1. Field of the Invention
This invention relates to vehicle suspension systems, and more particularly to a hydraulic actuator for a leveling system of a motor vehicle.
2. Description of Related Art
In recent years, substantial interest has grown in motor vehicle suspension systems which can offer improved comfort and road holding over the performance offered by conventional passive suspension systems. In general, such improvements are achieved by utilization of an "intelligent" suspension system capable of electronically controlling the suspension forces generated by hydraulic actuators provided at each corner of the motor vehicle.
Suspension systems are provided to filter or "isolate" the vehicle body from vertical road surface irregularities as well as to control body and wheel motion. In addition, it is desirable that the suspension system maintain an average vehicle attitude to promote improved platform stability during maneuvering. The classic passive suspension system includes a spring and a damping device in parallel which are located between the sprung mass (vehicle body) and the unsprung mass (wheel and axles).
Hydraulic actuators, such as shock absorbers and/or struts, are used in conjunction with convention passive suspension systems to absorb unwanted vibration which occurs during driving. To absorb this unwanted vibration, the hydraulic actuators often include a piston which is located within the actuator and is connected to the body of the automobile through a piston rod. Because the piston is able to limit the flow of damping fluid within the working chamber of the actuator when the actuator is telescopically displaced, the actuator is able to produce a damping force which counteracts the vibration which would otherwise be directly transmitted from the suspension to the vehicle body. The greater the degree to which the flow of damping fluid within the working chamber is restricted by the piston, the greater the damping forces which are generated by the actuator.
It is often desirable to have a leveling system which is associated with a shock absorber. Such leveling systems are used to change the height of the shock absorber, rather than to adjust the manner in which road vibration is transferred to the sprung portion of the automobile. In this regard, leveling systems are used to compensate for weight changes associated with the shock absorber which are the result of changes in two types of loading: static loading and dynamic loading. Static loading is simply the static load which is due to the weight associated with the passengers of the automobile, the weight of the cargo in the automobile, and so forth. In contrast, dynamic loading involves the loading which normally varies according to different types of road conditions.
Leveling systems of the type described above may be of varying construction. For example, U.S. Pat. No. 4,141,572 discloses a vehicle leveling system in which sensors are located in a pressurizable chamber and are used to sense the relative position of two movable members which are connected to the sprung and unsprung portion of the automobile, respectively. When the sensors indicate that the movable members are oriented proximate to one position which occurs when the shock absorber is very compressed, air of a relatively high pressure is delivered to the pressurizable chamber causing the shock absorber to elongate. When the sensors indicate that the movable members are oriented proximate to a second position which occurs when the shock absorber is overly extended, air is released from the pressurizable chamber so as to cause the shock absorber to contract.
Another type of vehicle leveling system is described in Lizell, M., "Dynamic Leveling for Ground Vehicles", Doctorial Thesis, Royal Institute of Technology, Stockholm, Sweden. In this reference, a dynamic leveling system for an automobile is described in which the actuator has both variable passive damping capability as well as leveling capability. Using this arrangement, the power consumption of the suspension system is relatively low.
Leveling systems of the type described above generally require the use of a pump for delivering relatively high pressure fluid to the hydraulic actuator. Because these systems have a single pump which delivers damping fluid to each of the shock absorbers, they often have one particular disadvantage. The use of a single pump often requires a fairly extensive hydraulic system which adds both cost and complexity to the leveling system.